Editing Polychlorinated dibenzodioxins (section)

==Sources of dioxins==
The [[United States Environmental Protection Agency]] inventory of sources of dioxin-like compounds is possibly the most comprehensive review of the sources and releases of dioxins,<ref name="epaSources">{{cite web|title=An Inventory of Sources and Environmental Releases of Dioxin-Like Compounds in the U.S. for the Years 1987, 1995, and 2000|url=http://oaspub.epa.gov/eims/eimscomm.getfile?p_download_id=459709|date= 2006-11-01|format=PDF}} EPA/600/P-03/002f, Final Report</ref> but other countries now have substantial research as well.

[[File:PCDD sediment core.svg|thumb|Concentration profile of PCDD in a dated sediment core from [[Esthwaite Water]], [[Cumbria]], [[UK]]]]

Occupational exposure is an issue for some in the [[chemical industry|chemical industries]], historically for those making [[chlorophenol]]s or [[Chlorophenoxy herbicide|chlorophenoxy acid herbicide]]s or in the application of chemicals, notably herbicides. In many developed nations there are now emissions regulations which have dramatically decreased the emissions<ref name="epaSources"/> and thus alleviated some concerns, although the lack of continuous sampling of dioxin emissions causes concern about the understatement of emissions. In [[Belgium]], through the introduction of a process called [[AMESA]], continuous sampling showed that periodic sampling understated emissions by a factor of 30 to 50 times. Few facilities have continuous sampling.

Dioxins are produced in small concentrations when organic material is [[combustion|burned]] in the presence of [[chlorine]], whether the chlorine is present as [[chloride]] [[ion]]s or as [[organochloride|organochlorine compound]]s, so they are widely produced in many contexts. According to the most recent US EPA data, the major sources of dioxins are broadly in the following types:<ref name="epaSources"/>

*Combustion sources, e.g. [[municipal waste]] or [[medical waste]] [[incinerator]]s and private backyard barrel burning
*[[Metal smelting]]
*Refining and process sources
*Chemical manufacturing sources
*Natural sources
*Environmental reservoirs

When first carried out in 1987, the original US EPA inventory of dioxin sources revealed that incineration represented more than 80% of known dioxin sources. As a result, US EPA implemented new emissions requirements. These regulations succeeded in reducing dioxin stack emissions from incinerators. Incineration of [[municipal solid waste]], [[medical waste]], [[sewage sludge]], and [[hazardous waste]] together now produce less than 3% of all dioxin emissions. Since 1987, however, [[Open burning of waste|backyard barrel burning]] has showed almost no decrease, and is now the largest source of dioxin emissions, producing about one third of the total output.<ref name="epaSources"/>

In incineration, dioxins can also reform or form ''de novo'' in the [[Earth's atmosphere|atmosphere]] above the stack as the exhaust gases cool through a temperature window of 600 to 200&nbsp;°C. The most common method of reducing the quantity of dioxins reforming or forming ''de novo'' is through rapid (30 millisecond) [[quenching]] of the exhaust gases through that 400&nbsp;°C window.<ref name="pmid17410797">{{cite journal | vauthors = Cheung WH, Lee VK, McKay G | title = Minimizing dioxin emissions from integrated MSW thermal treatment | journal = Environ Sci Technol | volume = 41 | issue = 6 | pages = 2001–7 | date = March 2007 | pmid = 17410797 | doi = 10.1021/es061989d | bibcode = 2007EnST...41.2001C }}</ref> Incinerator emissions of dioxins have been reduced by over 90% as a result of new emissions control requirements. Incineration in developed countries is now a very minor{{Citation needed|date=December 2022}} contributor to dioxin emissions.

Dioxins are also generated in reactions that do not involve burning&nbsp;— such as [[Bleaching of wood pulp#Chlorine and hypochlorite|chlorine bleaching]] fibers for [[paper]] or textiles, and in the manufacture of chlorinated phenols, particularly when reaction temperature is not well controlled.<ref>{{cite journal |author1=Kulkami P.S. |author2=Crespo J.G. |author3=Afonso C.A.M. | year = 2008 | title = Dioxins sources and current remediation technologies - a review | doi = 10.1016/j.envint.2007.07.009 |pmid=17826831 | journal = Environment International | volume = 34 | issue = 1| pages = 139–153 |bibcode=2008EnInt..34..139K }}</ref> Compounds involved include the wood preservative [[pentachlorophenol]], and also [[herbicide]]s such as [[2,4-D|2,4-dichlorophenoxyacetic acid]] (or 2,4-D) and [[2,4,5-trichlorophenoxyacetic acid]] (2,4,5-T). Higher levels of chlorination require higher reaction temperatures and greater dioxin production. Dioxins may also be formed during the [[photochemical]] breakdown of the common antimicrobial compound [[triclosan]].<ref name="pmid15779749">{{cite journal | vauthors = Latch DE, Packer JL, Stender BL, VanOverbeke J, Arnold WA, McNeill K | title = Aqueous photochemistry of triclosan: formation of 2,4-dichlorophenol, 2,8-dichlorodibenzo-p-dioxin, and oligomerization products | journal = Environ Toxicol Chem | volume = 24 | issue = 3 | pages = 517–25 | date = March 2005 | pmid = 15779749 | doi = 10.1897/04-243r.1 | s2cid = 19322739 }}</ref>